The effects of 3,4,3'-triiodo-l-thyronine on didehydroretinol synthesis by isolated coho salmon retinal pigment epithelial cells

Visual pigments and opsin expression in the juveniles of three species of fish (rainbow trout, zebrafish, and killifish) following prolonged exposure to thyroid hormone or retinoic acid

Thyroid hormone (TH) and retinoic acid (RA) are powerful modulators of photoreceptor differentiation during vertebrate retinal development. In the embryos and young juveniles of salmonid fishes and rodents, TH induces switches in opsin expression within individual cones, a phenomenon that also occurs in adult rodents following prolonged (12 week) hypothyroidism. Whether changes in TH levels also modulate opsin expression in the differentiated retina of fish is unknown. Like TH, RA is essential for retinal development, but its role in inducing opsin switches, if any, has not been studied. Here we investigate the action of TH and RA on single-cone opsin expression in juvenile rainbow trout, zebrafish, and killifish and on the absorbance of visual pigments in rainbow trout and zebrafish. Prolonged TH exposure increased the wavelength of maximum absorbance (λmax ) of the rod and the medium (M, green) and long (L, red) wavelength visual pigments in all fish species examined. However, unlike the opsin switch that occurred following TH exposure in the single cones of small juvenile rainbow trout (alevin), opsin expression in large juvenile rainbow trout (smolt), zebrafish, or killifish remained unchanged. RA did not induce any opsin switches or change the visual pigment absorbance of photoreceptors. Neither ligand altered cone photoreceptor densities. We conclude that RA has no effect on opsin expression or visual pigment properties in the differentiated retina of these fishes. In contrast, TH affected both single-cone opsin expression and visual pigment absorbance in the rainbow trout alevin but only visual pigment absorbance in the smolt and in zebrafish. The latter results could be explained by a combination of opsin switches and chromophore shifts from vitamin A1 to vitamin A2.

Tissue-specific distribution patterns of retinoids and didehydroretinoids in rainbow trout Oncorhynchus mykiss

Retinoids (vitamin A) are known to be involved in many key biological functions in mammals, such as embryonic development, reproduction or vision. Besides standard vitamin A forms, freshwater fish tissues contain high levels of didehydroretinoids or vitamin A(2) forms. However, the tissue distribution, metabolism and function of both standard and particularly the didehydroretinoids are still poorly known in fish. In this study, we have quantified the levels of retinoids, including retinol, retinaldehyde, retinyl palmitate and their corresponding didehydro forms, as well as the levels of the active polar retinoids all-trans-, 9-cis- and 13-cis-retinoic acid in distinct tissues of juvenile rainbow trout. Our results indicate that the liver is clearly the main retinoid storage tissue in juvenile rainbow trout. Didehydroretinoids were dominant over retinoids in all analyzed tissues with the exception of plasma. Additionally, significant differences among tissues were observed between retinoids and didehydroretinoids, such as differences in the ester profiles and the proportions between free and esterified forms, suggesting that mechanisms that favor the utilization or storage of one of the other groups of compounds might exist in fish. Our data also show the presence of polar retinoids in different tissues of fish at the fmol/g scale. Overall, this study clearly demonstrates the presence of tissue-specific patterns of accumulation of both polar and nonpolar retinoids in fish tissues. The biological relevance of these findings should be the focus of future studies.

Ontogenetic changes in photoreceptor opsin gene expression in coho salmon (Oncorhynchus kisutch, Walbaum)

Pacific salmonids start life in fresh water then migrate to the sea, after a metamorphic event called smoltification, later returning to their natal freshwater streams to spawn and die. To accommodate changes in visual environments throughout life history, salmon may adjust their spectral sensitivity. We investigated this possibility by examining ontogenetic and thyroid hormone (TH)-induced changes in visual pigments in coho salmon (Oncorhynchus kisutch, Walbaum). Using microspectrophotometry, we measured the spectral absorbance (quantified by lambda(max)) of rods, and middle and long wavelength-sensitive (MWS and LWS) cones in three age classes of coho, representing both freshwater and marine phases. The lambda(max) of MWS and LWS cones differed among freshwater (alevin and parr) and ocean (smolt) phases. The lambda(max) of rods, on the other hand, did not vary, which is evidence that vitamin A(1)/A(2) visual pigment chromophore ratios were similar among freshwater and ocean phases when sampled at the same time of year. Exogenous TH treatment long wavelength shifted the lambda(max) of rods, consistent with an increase in A(2). However, shifts in cones were greater than predicted for a change in chromophore ratio. Real-time quantitative RT-PCR demonstrated that at least two RH2 opsin subtypes were expressed in MWS cones, and these were differentially expressed among alevin, parr and TH-treated alevin groups. Combined with changes in A(1)/A(2) ratio, differential expression of opsin subtypes allows coho to alter the spectral absorbance of their MWS and LWS cones by as much as 60 and 90 nm, respectively. To our knowledge, this is the largest spectral shift reported in a vertebrate photoreceptor.

Ontogeny of the thyroid hormones and cortisol in the gilthead sea bream, Sparus aurata

The aim of the study was to elucidate the pattern of changes in the whole body concentrations of thyroid hormones (TH) and cortisol in eggs, pre-larvae, and larvae in the gilthead sea bream, Sparus aurata. The ontogeny of these hormones was related to specific morphological characteristics that characterize early development. TH and cortisol were detected in substantial amounts in the eggs of sea bream. T3 concentration in eggs was found to be higher than that of T4 (T3, 4.02+/-0.38 ng g-1; T4, 1.63+/-0.28 ng g-1), as it is often observed in marine teleosts. T4 decreased until hatching (T4, 1.01+/-0.58 ng g-1), while T3 reached a minimum at the time of first feeding (T3, 0.19+/-0.02 ng g-1). As development proceeded, both hormones displayed a similar pattern of changes with increasing concentrations until the completion of metamorphosis (T3, 15.84+/-2.92 ng g-1; T4, 9.78+/-2.39 ng g-1), indicating that the hypothalamo-adenohypophysial-thyroid axis begins to function soon after first feeding. The pattern of changes of cortisol (F) concentration was almost parallel to that of T4, starting from 0.83 ng g-1 in eggs. Minimum F concentration was observed at hatching (0.39+/-0.03 ng g-1) and maximum at melanophores and scale formation (14.82+/-2.71 and 14.82+/-3.12 ng g-1, respectively). Results provide data for the first time on the ontogeny of thyroid hormones and cortisol during the early development in sea bream and are in agreement with results in other fish species for an important action of TH during early development.